Reaction injection molding (RIM) is a process for molding polymeric parts by injecting low molecular weight, reactive, low viscosity liquids at high pressure into a mixing chamber and then into a mold cavity. The liquid reactants polymerize in the mold to form a molded article. Molds used for reaction injection molding, like many molds, comprise at least two segments, which in a mold-closed position, come together to define a mold cavity into which the reactants are injected. The mold segments have complimentary shaped faces which come together when the mold is closed to define a parting line. Unfortunately, there is usually some degree of mismatch between the faces. This mismatch of the mold segments results in the formation of small gaps between the mating faces at the parting line. Such gaps allow liquid mixture injected into the mold cavity to invade the parting line at its edges and produce protrusions, known as “flash”, on the finish article.
A particular application for reaction injection molding of a component directly on a substrate is in the production of acoustic barrier systems for motor vehicles. It is common practice in the motor vehicle industry to employ a double wall acoustic barrier arrangement to reduce noise from the engine compartment to the passenger compartment of a motor vehicle. The acoustic barrier system typically includes a metal wall separating the engine compartment from the passenger compartment, and a dash barrier spaced away from the passenger compartment side of the metal wall. A foam (i.e., an expanded thermoplastic material) or fiber sound absorbing or damping material is typically disposed in the space defined between the steel wall and the dash barrier. The dash barrier is typically a panel made of self-supporting, relatively dense, resilient or flexible synthetic plastic material, and the sound absorbing or damping material, also referred to as a decoupler, is typically in the form of a foam sheet or panel, or a fibrous mat. Often, the decoupler panel or mat is attached to the barrier panel prior to installation of the acoustic barrier and decoupler into a vehicle so that the barrier and decoupler are installed as a unit into a vehicle.
A premium process for producing high quality acoustic barrier systems comprising an acoustic barrier panel secured to a foam panel includes steps of injection molding an acoustic barrier panel, and forming the foam panel in a mold cavity directly against a surface of the barrier panel using a reaction injection molding (RIM) technique. Heretofore, it has not been possible to produce a foam RIM part without unwanted thin protrusions (flash) at the edges of the part which were located at the parting line of the mold segments. In order to meet the rigid dimensional and quality requirements common in the automotive industry, the finished parts must generally be free of flash.
Attempts to eliminate flash from RIM parts have generally focused on removing the flash after the molding process has been completed. Various mechanical flash removal apparatuses, cryogenic flash removal methods, and handwork processes have been devised to remove flash from RIM parts. However, these apparatuses and processes have achieved very limited success. Mechanical flash removal apparatuses have generally failed to consistently remove all of the flash material from a RIM part and/or have removed surface material from the RIM part resulting in destruction of texturing or other surface features of the part in the area of the mold seam. Similarly, cryogenic processing and/or handwork processing have not provided consistent satisfactory results due to incomplete removal of all flash material and/or damage to surface features of the RIM part. Further, handwork processes are very time consuming and labor intensive, with as much as about 40% of the total production cost for certain RIM parts being attributable to hand removal of flash material.
A flash-proof RIM mold and method are allegedly disclosed in U.S. Pat. No. 5,543,159. The method involves use of an interlayer film of thermosetting resin disposed on an edge of a parting line at one of a pair of mold segments. The interlayer film acts as a seal between the mating faces of the mold segments to fill any gaps between the mating faces and prevent intrusion of liquid reactants into the parting line during the RIM process. In order to cause the interlayer film to adhere firmly to the face of one mold segment and be readily separable from the face of the other mold segment, the face of a first mold segment is roughened to provide a multiplicity of anchoring sites for the film, and the face of a second mold segment is preferably polished to provide a smooth surface which will facilitate easy separation of the face of the second mold segment from the film. It is desirable to apply a mold release agent to the face of the second mold segment between each molding cycle to ensure release of the second mold segment from the interlayer film sealing the parting line gaps. The interlayer film is installed by depositing a strip or bead of thermosetting resin to the roughened surface of the first mold segment, closing the mold to allow the resin to flow and fill any gaps along the parting line, partially curing the resin while the mold is closed so as to form a material that is sufficiently hard to retain any formed contours after the mold is open, opening the mold, trimming away any material that has exuded into the mold cavity, and completing curing of the resin. It is alleged that the seal may be used to mold as many as about 5,000 parts before repair (i.e., replacement) of the seal is required.
Disadvantages with this process and apparatus include the requirement for modification of the mold segments, the need for applying mold release agents between each molding cycle, and the necessity for periodic removal of the film seal and fabrication of a new seal. Further, it is not evident how the process and apparatus may be adapted or modified for forming a RIM component directly on a thermoplastic substrate located in the mold cavity adjacent cavity surfaces of one of the mold segments.
Accordingly, there is a need for improved processes of reaction injection molding a component on a substrate while preventing or reducing formation of flash.